It has been proposed previously to provide an automatic focusing system, especially for use in focusing an image analysis system used to analyse the nature of cells present on a slide, which system effectively comprises a microscope.
Initial attempts at providing automatic focusing involved the use of a beam split attachment which split the image into three separate beams. Each respective part of the split beam was directed towards a respective image sensor. One sensor was located in the main focal plane of a relevant beam and the other two sensors were located respectively just short of and just behind the main focal plane of the respective beams. A focus error signal was then derived from the sensors, the signal consisting of a suitable non-linear function of illumination which favoured light/dark transitions at the edges of the objects and within objects. This signal was integrated for each sensor over the same field of view since, at any instant, the same image portion would be directed towards the three sensors. The integral would be a maximum for a sensor on which the image is sharply focused and the value of the integral provided by the two sensors which are equally out of focus would be the same. Consequently, the focusing of the arrangement could be adjusted so that the value of the integral derived from the main sensor would be at a maximum, and the value of the integrals obtained from the remaining two sensors would be equal.
Subsequently a modified automatic focusing arrangement was disclosed in European Patent Application 0163394A2. In this European Specification the focusing device comprises a sensor assembly having at least two arrays of sensor devices, with one array being in front of the optical image plane and the second array being behind the optical image plane. The sensor assembly has lines of pixel sensor elements. A differential contrast measure is taken from the pixel elements of the sensor which provides both the sign and magnitude of any focus error. This allows the focus motor to be driven in the correct direction. The differential contrast is calculated between one pixel and the next adjacent pixel. A contrast measure is taken from a selected number of pixels. The difference between the contracted pixel values will be greater for an image in focus than for a blurred image. The difference between the selected pixels is measured, and the large values are enhanced, for example by raising the measured values to a power. The measured values, when raised to a power may be summed to provide an indication of how well focused the object is. Exaggerating the large values is necessary because when considering an out of focus image, a number of small differences may sum to a value similar to a large single difference obtained from an image in focus. The image is moved relative to the sensor assembly (or the sensor is moved relative to the image) so that the same image area may be sequentially directed towards the two arrays. Signals are derived from the two arrays to generate a focus drive signal. The sensor of European Publication 0163394A2 is provided in addition to two further outputs of a microscope, one comprising a binocular eye-piece tube, and the other being in the form of a camera port, although the camera port is not usually active. Thus a beam splitter is required and three beams are created within the microscope, each with its own focus plane.
The present invention seeks to provide an improved automatic focusing device.